DIVERTED DEVELOPMENT OF REPRODUCTIVE-ORGANS - A SOURCE OF MORPHOLOGICAL INNOVATION IN LAND PLANTS

Recent discussions of animal development, particularly at the level of molecular genetics, have emphasized modularity, dissociation and co-o ption as basic principles of evolutionary developmental biology. These concepts are discussed in relation to two specific structural innovat ions in land plant evolution: the leaves (microphylls) of lycopsids, a nd the interseminal scales of Bennettitales. Both structures appear to have been derived evolutionarily by the diverted development of repro ductive organs. In the case of lycopsids, recent analyses of phylogene tic relationships suggest that leaves are sterilized sporangia modifie d for photosynthetic assimilation. In the case of Bennettitales, struc tural data suggest that the interseminal scales are sterilized ''cupul es'' modified for protection of the ovules. In both cases, multiplicat ion of reproductive organs seems to have accentuated functional redund ancy, and together with the developmental autonomy (dissociation) alre ady inherent in the modular construction of plants, appears to have fa cilitated sterilization and co-option of some of these structures for new purposes. Numerous other examples in plants illustrate the same pr inciples.